As you are already aware, the electrical conduction through the heart follows a set pathway under normal conditions.  Disturbances in these pathways will alter the pathway the wave of depolarization must follow and change the timing of the electrical events.  Some of these disturbances will produce visually obvious effects (see the next section:  a physiological approach to the abnormal ECG), while others will produce such subtle changes that only calculation of the actual time involved will clue you in.  In this section of the tutorial, we will cover the major calculations you will have to do in order to understand the ECG. 

Event/Interval/Segment Corresponds to: Normal range
PR interval* AV nodal delay 0.12-0.20 seconds
QRS duration* Ventricular depolarization up to 0.10 sec
QT interval* Total duration of ventricular depolarization (all myocytes) up to 0.43 sec (must be corrected for heart rate)
R-R interval (heart rate)* Time between beats - is used to calculate heart rate 0.6  to 1 sec

(heart rate:  60 - 100 bpm)

Mean Electrical Axis*(you will leave this page if you click here).  net vector of ventricular depolarization -30 - +110 degrees is the widest normal range (0  -  +90 is considered the normal by most)

Click on each to jump to that description on this page.  Those with asterisks by them are required calculations (fair game) for the section quiz or final.  Note that I.M. may require other calculations - this requirement is mine only. The mean electrical axis is considered in the next section.  

PR interval:  The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex.   It is a simple calculation!

Using our favorite ECG example:  First identify the beginning and end of what we are looking for. The PR interval goes from the beginning of the P wave to the beginning of the QRS complex:

Normpr.jpg (216209 bytes)

There are about 3 little squares between the beginning of the P wave and the beginning of the QRS complex (I just counted them out using the blue lines as a marker).  The only other piece of information that you need to know is that each little square represent 0.04 sec (25 mm/sec,  1 sq = 1mm so 1 sq = 1 /25 = 0.04). 

After that it is simple multiplication:    

0.04 sec / sq x 3 squares = 0.12 sec = PR interval in this person

(actually this is something of an underestimate because of the thickness of the lines I used to identify the PR interval)

 

The QRS duration:  The duration of the QRS complex is another useful (and simple)  calculation.  Here all we are going to do is measure the width of the QRS complex (how many squares) and multiply by our same 0.04.  

In our example: 

Calcqrs.jpg (53930 bytes)

In this example, there are roughly 2 squares between the beginning and the end of the QRS complex: 

The calculation is then: 

2 squares x 0.04 sec/square = 0.08 sec

Each QRS complex last about 0.08 seconds, perfectly normal. 

The QT interval:  This segment tells the total duration of the ventricular event, from when the first cell depolarizes to the repolarization of the last cell (which should be the same cell).  There are many drugs which alter the QT segment and a number of congenital diseases in which the QT segment is prolonged have now been identified (Long QT syndromes).  The Long QT syndromes are associated with an increased death rate from spontaneous ventricular arrhythmias in otherwise (apparently) healthy individuals.  The QT interval is very dependent on the heart rate, so you will often see the designation "QTc", denoting that the printed interval has been corrected for the heart rate. 

Calcqt2.jpg (54383 bytes)

   We follow the same steps that we have done for the previous calculations: 

1.  Count the number of squares:  12    

2.  Multiply the number of squares by the unit time per square: 

12 squares x 0.04 sec/square= 

0.48 seconds

According to our table that is a little long, but this is a calculation that requires a correction factor because the QT interval changes as heart rate changes.  The correction factor (a graph or one of two fancy equations)  puts the QT interval in this person within the borderline normal range. 

Calculation of the heart rate:  The R-R intervalThe RR interval is the time between QRS complexes.  The instantaneous heart rate can be calculated from the time between any two QRS complexes.  The drawback of this method is that the calculated heart rate can be quite a bit different from the measured pulse even in a normal person due to variations in the heart rate associated with respiration (the sinus arrhythmia).  Although the calculation of the RR interval is quite easy, deriving the heart rate from that requires a few extra steps (all shown below).

Calcrr.jpg (215407 bytes)

1.  Identify the landmarks (in consecutive beats) you will use for the calculation.  As shown above, I generally use the peak of the most obvious wave in the complex (In lead I, the R wave; in lead aVR, the Q wave). 

2.  Measure the distance between your landmarks.   Using lead I, I count 25 mm (squares) between the two peaks.

3.  Multiply the distance times the time scale (This will give you the RR interval):

25 mm/beat x 0.04 sec/sqare = 1 sec/beat = RR interval

The time between two R waves is one second.

4.  We currently know that 1beat takes 1 sec (i.e.   1 sec/beat).  In order to calculate the heart rate we need to have the numbers of beats/sec.   In order to do that, invert the RR interval (i.e. take 1/RR interval):

1/(1sec/beat) = 1 beat/sec

5.  Convert beats/sec into the more usual beats/min using the conversion factor:

1 beat/sec x 60 sec/min = 60 beats/min. 

There are many shortcuts for determining heart rate - any one of them is acceptable (they are all based on variations of this calculation).  

Skip Mean Electrical Axis and take a practice quiz. (this quiz also is linked at the end of the discussion on mean electrical axis).

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